alpha channel WHITE in animation
I suppose you are making the animation in the Video Sequence Editor so you could add a white strip in the background.


Shift + A &rarr; Effect strip &rarr; Color.
In its properties (n) go to Effect strip section an change the Color to white.
Render again.


Maybe you need to select the Alpha Over mode in Blend option of the strip with your image sequence so it uses their alpha channel.